Antennas are often installed on conducting surfaces that are usually called ground planes. In many applications the ground plane is finite and is often terminated by an edge in the form of a sharp bend or corner. In the limiting case where the included angle of the bend goes to zero, there is a knife edge or half-plane. A radiating antenna will usually excite TM (transverse magnetic relative to the plane of incidence) mode waves which will travel along the ground plane with the Electric field (E field) normal to the surface until an edge reached. Then the TM wave will diffract, resulting in electromagnetic power being scattered into shadow regions, such as below the antenna ground plane. Such radiation into the shadow region is known as a backlobe. Edge diffraction may also result in increased side lobe levels (SLL) for directive antennas, when compared with a case where the ground plane is substantially infinite in planar size.
Edge diffraction is also responsible for a certain amount of spill-over loss in feed antennas (horns or patch arrays) for reflector, lens, or other quasi-optical antenna systems.
One means of suppressing edge diffraction for half-planes is to use a tapered periodic surface (TPS). This is a class of patterned, quasi-periodic, conductive surface where the period changes with distance from the edge such that the surface impedance gradually transforms from the essentially a zero surface impedance of a good ground plane to an infinite surface impedance beyond the edge. (TPS are described by Munk in section 9.6, Finite Antenna Arrays and FSS, 2003, John Wiley and Sons. Also, see U.S. Pat. No. 5,606,335 by Errol K. English et al.) A TPS generally requires a dedicated area along the edge whose width is a minimum of two or more wavelengths. Many antennas reside on very small ground planes where there is not enough space to use a TPS. A TPS can be used for a half-plane and not, for example, on a conducting wedge of non-zero included angle.
Resistive cards (R-card) have sometimes been used at edges of conductive ground planes to mitigate diffraction. However, the R-card material must be located at least one half of a free-space wavelength away from the edge of the antenna to avoid degradation of antenna radiation efficiency. Furthermore, R-card treatments must be augmented with volumetric absorbers (so-called radar absorbing material) in the case where the ground plane edge is not a half-plane but a corner with non-zero dihedral angle.
Magnetically-loaded radar absorbing material (MAGRAM) has been used at edges to suppress edge diffraction. However, this material is also RF lossy as it is composed of an iron or ferrite loaded insulator such as rubber or silicone. It is relatively heavy, and it cannot be used in the near field of an antenna without degrading the antenna radiation efficiency.
There exist certain situations where the enhancement of the diffraction coefficient is needed to improve the electromagnetic coupling around a corner. For instance, this may be desirable to obtain a more omni-directional antenna pattern for a communication antenna mounted on the side of a building. None of the above methods (TPS, R-card, or MAGRAM) will enhance the diffraction coefficient at an edge.